1. Field of the Invention
The present invention relates to an apparatus for measuring a signal from a vital or a living body for a predetermined period of time, and in particular, to a living body signal measuring apparatus for analyzing a signal vital in a realtime fashion.
2. Description of the Prior Art
In the field of medical treatment, a clinical examination measuring, for example, an electrocardiogram and blood pressure is conducted as a daily job.
These examinations are ordinarily effected in quite a limited period of time in a medical care institution such as a clinic or a hospital under contro of a medical doctor or an examiner or inspector of the clinical institution. However, there exist diseases which cannot be detected in the examination conducted in such a limited period of time.
For example, in a case of a temporary disorder of the heart or an arrhythmia, there does not necessarily appear an abnormal waveform in the electrocardiogram. Since an interval between appearances of such an abnormal waveform is great in many cases, the probability of the detection of such a disorder of the heart in a short period of time is decreased; in consequence, it is difficult in many cases to conduct a determinant diagnosis through a examination conducted in a short period of time.
In this situation, for the detection of such diseases described above, there has been devised a method for measuring an electrocardiogram for a long period of time. According to this method, there is attained a long-term elctrocardiogram or so-called Holter electrocardiogram such that for an entire day or 24 hours in the daily life, a portable electrocardiograph is kept attached on a portion of the body of a person as a subject so as to collect and to record electrocardiographic waveforms on a magnetic tape. The magnetic tape is thereafter read by use of a magnetic tape playback apparatus so as to reproduce the electrocardiographic waveforms, which are observed by an inspector such as a medical doctor to detect an abnormality, thereby conducting a diagnosis of disease such as a fugitive affection of the heart.
The electrocardiographic waveforms recorded on a magnetic tape are reproduced at a high speed due to the great volume thereof. Consequently, the inspector, for example, a medical doctor is required to conduct the diagnosis by visually checking such a great amount of electrocardiographic waveforms reproduced at a high speed, which imposes a heavy load on the inspector.
As described above, in order to detect the arrhythmia which is a transitory disease and which appears at quite a rare occasion, the electrocardiographic waveforms collected in 24 hours are entirely reproduced so as to analyze the waveforms in a long period of time, which requires many unnecessary jobs and which decreases the efficiency of the overall examination.
In order to eliminate the irrationality above, there has been adopted a method in which a magnetic tape containing a great volume of recorded waveforms is reproduced at a high speed (for example, at a speed which is 60 or 120 times the ordinary playback speed) such that the waveforms are automatically analyzed according to a predetermined method by the apparatus so as to display only the portions thereof judged to be abnormal as a result of the analysis, thereby enabling an inspector to examine the abnormal portions. However, since the magnetic tape undergoes a high-speed playback operation and data items of the electrocardiographic waveforms are inputted to the objective apparatus at a high speed, when it is desired to analyze the waveforms by use of, for example, a microcomputer, there cannot be afforded a sufficient time for the analysis, namely, it is difficult at the present stage of the technology to increase the accuracy of the analysis.
In order to overcome such a difficulty, there has been recently employed a method in which while the electrocardiographic waveforms are being gathered for a long period of time, an analysis of the waveforms are automatically achieved at the same time, namely, in a realtime manner such that portions of the waveforms judged to be abnormal as a result of the analysis are stored in a storage, for example, on a magnetic tape or in an IC memory so as to thereafter display these abnormal waveforms and analysis results by means of a display equipment installed at a location of a medical doctor and to effect a print-out operation thereof if necessary, which enables the doctor to confirm the results. In this method, as compared with the method above, a sufficient time can be afforded to conduct the analysis (60 or 120 times), which improves the accuracy of the analysis.
In the realtime analysis of such a long-term measurement of electrocardiographic waveforms, all electrocardiographic waveforms appearing in 24 hours are not recorded, namely, there are stored only the waveforms judged to be abnormal as a result of the automatic analysis effected by the apparatus. In consequence, the waveforms judged to be normal by the automatic analysis are not stored, namely, even if there exists an abnormal waveform therein, the inspector cannot check such a waveform.
As described above, in the realtime analysis, it is quite important that any abnormal waveform can be detected; in addition, in order to minimize the examination job imposed on the inspector, it is also essential not to judge a normal waveform to be abnormal.
However, since the electrocardiographic waveforms include personal characteristics of the subject (differences in characteristcs associated with respective persons), if the normality and the abnormality are automatically judged depending on a predetermined reference value, it is likely to increase the ratio of errors in the judgement.
In other words, when using electrocardiographic waveforms, a QRS portion where a level of the waveform signal greatly varies is detected so as to compute values of an area of the QRS portion, an amplitude (height) thereof, and the time index (index of the width) thereof, thereby judging to determine the normality or the abnormality depending on whether or not these values exceed the respective predetermined threshold values. Due to the difference between the threshold values of the individual persons, with any threshold values set in a case where the realtime analysis is effected while continuously measuring the signal for 24 hours, there remains a fear that the error ratio is increased in the judgement.
For example, an attempt has been made to correctly detect with a high accuracy an extrasystole associated with the ventricule which is most problematical in the arrhythmia. In this case, the time index of the QRS portion (the index of the QRS width) is measured so as to determine the presence or absence of the extrasystole associated with the ventricule depending on whether or not the value of the measured index exceeds a predetermined threshold value. However, the threshold value as a boundary value between the normal value of the QRS index and the abnormal value thereof cannot be absolutely determined, namely, the value varies depending on the individual cases; moreover, it has been well known that, for a person, the value changes during a day or between respective examination days. Namely, even when the same value is obtained for the QRS index, the value may be normal or abnormal depending on the individual cases. In consequence, it is impossible to uniquely fix the threshold value to judge the normal and abnormal cases without causing any errors.